Simultaneously measurements of the PVT and thermal – pressure coefficient of benzene in the critical and supercritical regions

Abstract

The values of pressure (P), the temperature derivative, γV = (∂P/∂T)V, and the density (ρ) of benzene have been simultaneously measured in the near- and supercritical regions using high-temperature and high-pressure piezo-calorimeter. Measurements were made along 10 liquid and vapor isochores between (265.5 and 653.9) kg·m−3 and at temperatures from (346.03 to 615.92) K and at pressures up to 9.171 MPa. For each isochore most measurements were made in the immediate vicinity of the liquid-gas phase transition temperature (single- and two-phase regions) where the break of the P-T isochores and the jumps of the thermal –pressure coefficient γV are observing. Temperatures and pressures (TS, PS) at the liquid-gas phase transition curve for each constant density (isochore, ρ) and the critical parameters (TC,PC,ρC) for benzene were measured using the isochoric P − T break point and thermal –pressure coefficient jump techniques. The expanded uncertainty of the pressure (P), density (ρ), and thermal –pressure coefficient (γV) measurements at the 95% confidence level with a coverage factor of k = 2 is estimated to be, 0.16%, 0.05% and (0.12 to 1.5) % (depending on temperature and pressure), respectively. The measured pressures (PVT) and thermal –pressure coefficients (γVVT) have been used to calculate of the internal pressure (or energy-volume coefficient) as (∂U∂V)T=T(∂P∂T)V−P. We have also measured the temperature derivatives of the internal energy (∂U∂T)V=CV (isochoric heat capacity) using the same piezo-calorimetric cell. The effect of pressure and temperature on the internal pressure was studied. Isochoric P-T curve break point and isochoric thermal-pressure coefficient jump techniques were used to accurately determine of the phase transition and critical points parameters. The measured values of the thermal-pressure coefficient were interpreted in term of scaling theory of critical phenomena. The measured values of pressure (P), temperature derivative, (∂P/∂T)V, temperature and density at the saturation curve together with our previous isochoric heat capacity measurements were used to calculate other thermodynamic properties of benzene at saturation curve.